Fluid check valves in general are well known in the art and generally are of two types. The first type incorporates a valve seat and ball or valve headed stem spring-biased against the seat to a closed position, the same being forced open when a given differential pressure across the valve is exceeded. When this differential pressure decreases or reverses, the ball or valve stem head will seat on the valve seat and block reverse flow.
The second types of fluid check valves are generally of an integral construction of elastomeric material, shaped to define a lip or flap which will open when a given differential pressure across the body is exceeded and close when this differential pressure reverses.
One of the problems with the second type of valve described above is its tendency to flutter when the pressure differentials fall within certain ranges. Accordingly, efforts have been made to attempt to control such flutter by utilizing different types of elastomeric material; that is, material having different elastic properties. However, there are only certain elastomeric materials which can be molded and serve satisfactorily for such valves and thus in some instances it has been necessary to resort to the first type of valves described above which are more expensive to manufacture.